Dielectric structure overlying image sensor element to increase quantum efficiency

ABSTRACT

Various embodiments of the present disclosure are directed towards a pixel sensor. The pixel sensor includes a substrate having a front-side opposite a back-side. An image sensor element comprises an active layer disposed within the substrate, where the active layer comprises germanium. An anti-reflective coating (ARC) structure overlies the back-side of the substrate. The ARC structure includes a first dielectric layer overlying the back-side of the substrate, a second dielectric layer overlying the first dielectric layer, and a third dielectric layer overlying the second dielectric layer. A first index of refraction of the first dielectric layer is less than a second index of refraction of the second dielectric layer, and a third index of refraction of the third dielectric layer is less than the first index of refraction.

REFERENCE TO RELATED APPLICATION

This Application is a Divisional of U.S. application Ser. No.17/197,291, filed on Mar. 10, 2021, the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND

Many modern day electronic devices (e.g., digital cameras, opticalimaging devices, etc.) comprise image sensors. Image sensors convertoptical images to digital data that may be represented as digitalimages. An image sensor includes an array of pixel sensors, which areunit devices for the conversion of an optical image into digital data.Some types of pixel sensors include charge-coupled device (CCD) imagesensors and complementary metal-oxide-semiconductor (CMOS) image sensors(CIS). Compared to CCD pixel sensors, CIS are favored due to low powerconsumption, small size, fast data processing, a direct output of data,and low manufacturing cost.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a cross-sectional view of some embodiments of a pixelsensor having an anti-reflective coating (ARC) structure overlying animage sensor element, where the ARC structure is configured to enhancethe quantum efficiency (QE) of the image sensor element.

FIGS. 2A-2F illustrate cross-sectional views of some embodiments ofpixel sensors according to some alternative embodiments of the pixelsensor of FIG. 1 .

FIG. 3 illustrates a cross-sectional view of some embodiments of anintegrated circuit (IC) comprising a first IC die underlying a second ICdie, where the second IC die includes an ARC structure overlying aplurality of image sensor elements.

FIGS. 4-11 illustrate cross-sectional views of some embodiments of amethod of forming a pixel sensor having an ARC structure overlying animage sensor element.

FIG. 12 illustrates a methodology in flowchart format that illustratessome embodiments of forming a pixel sensor having an ARC structureoverlying an image sensor element.

DETAILED DESCRIPTION

The present disclosure provides many different embodiments, or examples,for implementing different features of this disclosure. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Moreover, “first”, “second”, “third”, etc. may be used herein for easeof description to distinguish between different elements of a figure ora series of figures. “first”, “second”, “third”, etc. are not intendedto be descriptive of the corresponding element, but rather are merelygeneric identifiers. For example, “a first dielectric layer” describedin connection with a first figure may not necessarily correspond to a“first dielectric layer” described in connection with some embodiments,but rather may correspond to a “second dielectric layer” in otherembodiments.

CMOS image sensors (CIS) typically comprise an array of pixel regions,which respectively have an image sensor element arranged within asubstrate. An anti-reflective coating (ARC) structure overlies the imagesensor elements and is configured to prevent reflection of incidentlight away from the substrate. Upon receiving light, the image sensorelements are configured to generate electric signals corresponding tothe received light. The electric signals from the image sensor elementscan be processed by a signal processing unit to determine an imagecaptured by the CIS. Quantum efficiency (QE) is a ratio of the numbersof photons that contribute to an electric signal generated by an imagesensor element within a pixel region to the number of photons incidenton the pixel region. It has been appreciated that the ARC structure mayenhance QE of a CIS by preventing the reflection of incident light awayfrom underlying image sensor elements.

One challenge with the above pixel sensor is reflection of the incidentradiation away from an underlying image sensor element. In someembodiments, the ARC structure comprises a first dielectric layerdisposed over the underlying image sensor element and a seconddielectric layer overlying the first dielectric layer. The firstdielectric layer has a first index of refraction that is greater than asecond index of refraction of the second dielectric layer (e.g., thefirst index of refraction is about 2 and the second index of refractionis about 1.44), such that a difference between the first and secondindices of refraction is relatively small (e.g., less than about 0.6).Due to the difference between the first and second indices ofrefraction, incident light that passes from the second dielectric layerto the first dielectric layer is bent towards a line that is normal toan upper surface of the first dielectric layer, thereby directing thelight towards the underlying image sensor element. However, because thedifference between the first and second indices of refraction isrelatively small (e.g., less than about 0.6), the incident light is notsufficiently bent towards the line. This may result in the incidentlight traveling to an adjacent image sensor element and/or beingreflected away from the underlying image sensor element, therebydecreasing a QE of the pixel sensor, increasing cross-talk, anddecreasing an overall performance of the pixel sensor.

In some embodiments, the present application is directed towards a pixelsensor having an anti-reflective coating (ARC) structure overlying animage sensor element and configured to enhance a QE of the pixel sensor.For example, the pixel sensor comprises an image sensor element disposedwithin a substrate. The ARC structure overlies a back-side of thesubstrate and comprises a first dielectric layer, a second dielectriclayer, and third dielectric layer. The first dielectric layer overliesthe back-side of the substrate, the second dielectric layer overlies thefirst dielectric layer, and the third dielectric layer overlies thesecond dielectric layer. The first dielectric layer comprises a firstindex of refraction, the second dielectric layer comprises a secondindex of refraction greater than the first index of refraction, and thethird dielectric layer comprises a third index of refraction less thanthe second index of refraction (e.g., the second index of refraction isabout 2.43 and the third index of refraction is about 1.44). Thus, adifference between the second and third indices of refraction isrelatively large (e.g., greater than about 0.6). Due to the relativelylarge difference between the second and third indices of refraction,incident light that passes from the third dielectric layer to the seconddielectric layer is sufficiently bent towards a line that is normal toan upper surface of the second dielectric layer, thereby sufficientlydirecting the incident light towards the underlying image sensorelement. Thus, the incident light being directed towards an adjacentimage sensor element and/or being reflected away from the underlyingimage sensor element is mitigated. This increases a QE of the imagesensor element and decreases cross-talk, thereby increasing an overallperformance of the pixel sensor.

FIG. 1 illustrates a cross-sectional view of some embodiments of a pixelsensor 100 having an anti-reflective coating (ARC) structure 118overlying a substrate 110. In some embodiments, the circle 140illustrates an enlarged view of a region of the pixel sensor 100 to moreclearly illustrate details of the ARC structure 118.

The pixel sensor 100 includes an interconnect structure 102 disposedalong a front-side 110 f of the substrate 110. An image sensor element112 is disposed within the substrate 110. The image sensor element 112is configured to convert electromagnetic radiation 130 (e.g., photons)into electrical signals (e.g., to generate electron-hole pairs from theelectromagnetic radiation 130). In some embodiments, the electromagneticradiation 130 is back side illuminated (BSI) upon the pixel sensor 100.In some embodiments, the image sensor element 112 may, for example, beconfigured to generate electrical signals from near infrared (NIR)radiation (e.g., electromagnetic radiation with wavelengths in a rangeof about 700 nanometers (nm) to about 3,000 nm).

In some embodiments, the substrate 110 is comprised of a first material(e.g., silicon). Further, the image sensor element 112 comprises anactive layer 114 that is disposed within the substrate 110. In furtherembodiments, the active layer 114 comprises a second material (e.g.,germanium) that is different from the first material. A first isolationstructure 116 is disposed within the substrate 110 and provideselectrical isolation to the image sensor element 112 from other devicesand/or doped regions disposed within/on the substrate 110. The firstisolation structure 116 may laterally enclose the image sensor element112. The active layer 114 may, for example, comprise photodetectorregions and/or layers such as charge storage region(s), floatingnode(s), surface pinning regions(s), contact region(s), guard ring(s),etc. configured to convert electromagnetic radiation 130 (e.g., photons)into electrical signals and/or facilitate readout of the electricalsignals. The second material of the active layer 114 is selected toensure high QE for NIR radiation and/or infrared radiation (IR). Forexample, the second material (e.g., germanium) of the active layer 114aides in absorption of IR radiation by the active layer 114, therebyincreasing a QE of the image sensor element 112.

The interconnect structure 102 extends along a front-side 110 f of thesubstrate 110 and is configured to electrically couple doped regions ofthe substrate 110 and/or the active layer 114 and pixel devices (e.g.,transfer transistor(s), source-follower transistor(s), row-selecttransistor(s), etc.) to one another. The interconnect structure 102includes an interconnect dielectric structure 104, a plurality ofconductive wires 106, and a plurality of conductive vias 108. Further,the ARC structure 118 is disposed along a back-side 110 b of thesubstrate 110. The ARC structure 118 comprises a first dielectric layer120 with a first refractive index, a second dielectric layer 122 with asecond refractive index, and a third dielectric layer 124 with a thirdrefractive index. Further, a grid structure 126 overlies the ARCstructure 118. The grid structure 126 may, for example, comprise a metalgrid structure and/or a dielectric grid structure. The grid structure126 is configured to direct the electromagnetic radiation 130 to theunderlying image sensor element 112. In some embodiments, when the gridstructure 126 comprises the metal grid structure (e.g., aluminum,copper, tungsten, or a combination of the foregoing), electromagneticradiation 130 may reflect off of sidewalls of the metal grid structureto the underlying image sensor element 112 instead of traveling to anadjacent image sensor element (not shown). In such embodiments, the gridstructure 126 may decrease cross-talk between adjacent image sensorelements, thereby increasing the QE of the image sensor element 112. Inaddition, an upper dielectric layer 128 is disposed over the gridstructure 126 and the ARC structure 118.

In various embodiments, the electromagnetic radiation 130 is disposedupon the ARC structure 118 and comprises a first range of wavelengths.In some embodiments, the first dielectric layer 120 may, for example, beor comprise tantalum oxide (e.g., Ta₂O₅), another dielectric material,or any combination of the foregoing and/or the first index of refractionmay be about 2, about 2.06, within a range of about 2 to 2.1, or anothersuitable value. In some embodiments, if the first range of wavelengthsis within a range of about 400 nm to about 700 nm (e.g., visible light),then the first index of refraction may be within a range of about 2.11to about 2.25, or another suitable value. In further embodiments, if thefirst range of wavelengths is within a range of about 700 nm to about3,000 nm (e.g., NIR radiation), then the first index of refraction maybe within a range of about 2.01 to 2.11, or another suitable value. Invarious embodiments, if the first range of wavelengths is within a rangeof about 1,400 nm to about 1,600 nm, then the first index of refractionmay be within a range of about 2.056 to about 2.062, or another suitablevalue.

In various embodiments, the second dielectric layer 122 may, forexample, be or comprise titanium oxide (e.g., TiO₂), silicon carbide,another suitable dielectric material, or any combination of theforegoing and/or the second index of refraction may be about 2.43,within a range of about 2.4 to 2.6, or another suitable value. In someembodiments, if the first range of wavelengths is within a range ofabout 400 nm to about 700 nm (e.g., visible light), then the secondindex of refraction may be within a range of about 2.55 to about 2.88,or another suitable value. In further embodiments, if the first range ofwavelengths is within a range of about 700 nm to about 3,000 nm (e.g.,NIR radiation), then the second index of refraction may be within arange of about 2.37 to 2.55, or another suitable value. In variousembodiments, if the first range of wavelengths is within a range ofabout 1,400 nm to about 1,600 nm, then the second index of refractionmay be within a range of about 2.43 to about 2.46, or another suitablevalue.

In some embodiments, the third dielectric layer 124 may, for example, beor comprise silicon dioxide (e.g., SiO₂), another oxide, high densityplasma oxide, another suitable dielectric material, or any combinationof the foregoing and/or the third index of refraction may be about 1.44,about 1.438, about 1.53, within a range of about 1.438 to 1.53, oranother suitable value. In some embodiments, if the first range ofwavelengths is within a range of about 400 nm to about 700 nm (e.g.,visible light), then the third index of refraction may be within a rangeof about 1.46 to about 1.49, or another suitable value. In furtherembodiments, if the first range of wavelengths is within a range ofabout 700 nm to about 3,000 nm (e.g., NIR radiation), then the thirdindex of refraction may be within a range of about 1.42 to 1.46, oranother suitable value. In various embodiments, if the first range ofwavelengths is within a range of about 1,400 nm to about 1,600 nm, thenthe third index of refraction may be within a range of about 1.443 toabout 1.446, or another suitable value. In various embodiments, thesecond index of refraction is greater than the first index of refractionand the third index of refraction is less than the first index ofrefraction. This, in part, decreases reflection of electromagneticradiation away from the image sensor element 112, thereby increasing aQE of the pixel sensor 100.

In some embodiments, the arrow 130 a illustrates some non-limitingexamples of a path of the electromagnetic radiation 130 as it travelsthrough the ARC structure 118. As the electromagnetic radiation 130traverses a boundary between the third dielectric layer 124 and thesecond dielectric layer 122 it is bent towards a first normal line 132.Because the third index of refraction is substantially less than thesecond index of refraction (i.e., a difference between the second andthird indices of refraction is greater than about 0.6), a first angle ofrefraction θ2 is substantially less than a corresponding angle ofincidence θ₁, thereby focusing the electromagnetic radiation 130 towardsthe image sensor element 112. This mitigates reflection of theelectromagnetic radiation 130 away from the image sensor element 112and/or decreases cross-talk, thereby increasing the QE of the pixelsensor 100. Further, as the electromagnetic radiation 130 traverses aboundary between the second dielectric layer 122 and the firstdielectric layer 120 it is bent away from a second normal line 134.Because the first index of refraction is relatively less than the secondindex of refraction (i.e., a difference between the second and firstindices of refraction is less than about 0.5), a second angle ofrefraction θ3 is slightly greater than the first angle of refraction θ₂.Because the difference between the second and first indices ofrefraction is relatively small, reflection of the electromagneticradiation away from the image sensor element 112 is mitigated. Finally,in some embodiments, an index of refraction of the substrate 110 isgreater than the first index of refraction (e.g., the index ofrefraction of the substrate 110 is within a range of about 3.42 to 3.48or another suitable value), such that as the electromagnetic radiationpasses from the first dielectric layer 120 to the substrate 110 it isbent towards a line normal to the back-side 110 b of the substrate 110.This directs the electromagnetic radiation 130 towards the image sensorelement 112, and mitigates reflection of the electromagnetic radiation130 away from the image sensor element 112 and/or decreases cross-talk,thereby increasing the QE of the pixel sensor 100. Thus, the ARCstructure 118 is configured to increase a performance of the pixelsensor 100. In some embodiments, the first normal line 132 is normal toan upper surface of the second dielectric layer 122 (an interfacebetween the third dielectric layer 124 and the second dielectric layer122), the second normal line 134 is normal to an upper surface of thefirst dielectric layer 120 (an interface between the second dielectriclayer 122 and the first dielectric layer 120), and/or the first andsecond normal lines 132, 134 are parallel to one another.

FIG. 2A illustrates a cross-sectional view of some embodiments of apixel sensor 200 a according to some alternative embodiments of thepixel sensor 100 of FIG. 1 .

The pixel sensor 200 a includes an interconnect structure 102 disposedalong a front-side 110 f of a substrate 110 and an ARC structure 118disposed along a back-side 110 b of the substrate 110. In someembodiments, the substrate 110 may, for example, be or comprise a bulksubstrate (e.g., a bulk silicon substrate), a silicon-on-insulator (SOI)substrate, P-doped silicon, or another suitable material. Thus, thesubstrate 110 comprises a first material such as, for example, silicon.The interconnect structure 102 comprises an interconnect dielectricstructure 104, a plurality of conductive wires 106, and a plurality ofconductive vias 108. In further embodiments, the interconnect dielectricstructure 104 may, for example, be or comprise a silicon dioxide, alow-k dielectric material, another suitable dielectric material, or anycombination of the foregoing. In yet further embodiments, the conductivewires 106 and/or the conductive vias 108 may, for example, respectivelybe or comprise aluminum, copper, ruthenium, tungsten, titanium nitride,tantalum nitride, another suitable material, or any combination of theforegoing. The interconnect structure 102 is configured to electricallycouple doped regions and/or semiconductor devices disposed within thepixel sensor 200 a to one another.

An image sensor element 112 is disposed within the substrate 110 andcomprises an active layer 114. The image sensor element 112 is, forexample, configured to generate electrical signals from near infrared(NIR) radiation (e.g., electromagnetic radiation with wavelengths in arange of about 700 nanometers (nm) to about 3,000 nm). It will beappreciated that the image sensor element 112 being configured togenerate electrical signals from other frequency wavelength values isalso within the scope of the disclosure. A first isolation structure 116is disposed within the substrate 110 and laterally encloses the imagesensor element 112. The first isolation structure 116 extends from thefront-side 110 f of the substrate 110 to a point above the front-side110 f of the substrate 110. The first isolation structure 116 isconfigured to electrically isolate the image sensor element 112 fromother devices disposed on and/or within the substrate 110. In someembodiments, the first isolation structure 116 is configured as ashallow trench isolation (STI) structure or another suitable isolationstructure. In further embodiments, the first isolation structure 116may, for example, be or comprise an oxide, such as silicon dioxide,silicon nitride, silicon carbide, silicon oxynitride, siliconoxycarbide, another suitable dielectric material, or any combination ofthe foregoing.

Further, a second isolation structure 202 extends from the back-side 110b of the substrate 110 to a point below the back-side 110 b of thesubstrate 110. In some embodiments, a bottom surface of the secondisolation structure 202 may contact a top surface of the first isolationstructure 116. The second isolation structure 202 laterally surroundsthe image sensor element 112 and is configured to electrically isolatethe image sensor element 112 from other devices disposed within and/oron the substrate 110. In some embodiments, the second isolationstructure 202 is configured as a deep trench isolation (DTI) structureor another suitable isolation structure. Further, in some embodiments,the second isolation structure 202 may, for example, be or comprise adielectric material (e.g., silicon dioxide, silicon nitride, siliconcarbide, another dielectric material, or any combination of theforegoing), a metal material (e.g., tungsten, copper, aluminum, anothermetal, or any combination of the foregoing), another suitable material,or any combination of the foregoing. In further embodiments, the secondisolation structure 202 may be configured to direct incidentelectromagnetic radiation towards the underlying image sensor element112. For example, when the second isolation structure 202 comprises ametal material (e.g., aluminum, copper, tungsten, etc.), electromagneticradiation may reflect off of sidewalls of the metal material to theunderlying image sensor element 112 instead of traveling to an adjacentimage sensor element (not shown). In such embodiments, the secondisolation structure 202 may decrease cross-talk between adjacent imagesensor elements, thereby increasing a QE of the image sensor element112.

The ARC structure 118 is disposed along the back-side 110 b of thesubstrate 110. The ARC structure 118 includes a first dielectric layer120 with a first index of refraction, a second dielectric layer 122 witha second index of refraction, and a third dielectric layer 124 with athird index of refraction. In some embodiments, the first dielectriclayer 120 may, for example, be or comprise tantalum oxide (e.g., Ta₂O₅),another dielectric material, or any combination of the foregoing and/orthe first index of refraction may be about 2, about 2.06, within a rangeof about 2 to 2.16, or another suitable value. In further embodiments,the second dielectric layer 122 may, for example, be or comprisetitanium oxide (e.g., TiO₂), silicon carbide, another suitabledielectric material, or any combination of the foregoing and/or thesecond index of refraction may be about 2.43, within a range of about2.4 to 2.6, or another suitable value. In yet further embodiments, thethird dielectric layer 124 may, for example, be or comprise silicondioxide (e.g., SiO₂), another oxide, high density plasma oxide, anothersuitable dielectric material, or any combination of the foregoing and/orthe third index of refraction may be about 1.44, about 1.438, about1.53, within a range of about 1.438 to 1.53, or another suitable value.In various embodiments, the second index of refraction is greater thanthe first index of refraction and/or the third index of refraction isless than the first index of refraction. This, in part, decreasesreflection of electromagnetic radiation away from the image sensorelement 112, thereby increasing a QE of the pixel sensor 200 a.

The first dielectric layer 120 has a first thickness t1, the seconddielectric layer 122 has a second thickness t2, and the third dielectriclayer 124 has a third thickness t3. In some embodiments, the firstthickness t1 is, for example, about 1,100 Angstroms, within a range ofabout 900 to 1,300 Angstroms, or another suitable thickness value. Infurther embodiments, the second thickness t2 is, for example, about 200Angstroms, within a range of about 10 to 1,000 Angstroms, or anothersuitable thickness value. In yet further embodiments, the thirdthickness t3 is, for example, about 1,300 Angstroms, within a range ofabout 1,100 to 1,500 Angstroms, or another suitable thickness value. Invarious embodiments, the first thickness t1 is greater than the secondthickness t2, and the first thickness t1 is less than the thirdthickness t3.

In some embodiments, the image sensor element 112 is configured togenerate electrical signals from a first range of NIR radiation, wherethe first range of NIR radiation includes wavelengths within a range ofabout 1400 nm to 1600 nm, however, other values for the first range ofNIR radiation are within the scope of the disclosure. In someembodiments, if the second thickness t2 is less than about 10 Angstroms,then reflectance of incident electromagnetic radiation within the firstrange of NIR radiation is increased, thereby decreasing a performance ofthe pixel sensor 200 a. In yet further embodiments, if the secondthickness t2 is equal to or greater than about 200 Angstroms, thenreflectance of incident electromagnetic radiation within the first rangeof NIR radiation is decreased, thereby increasing a performance of thepixel sensor 200 a. In yet further embodiments, if the first range ofNIR radiation has a wavelength of about 1550 nm and the second thicknesst2 is approximately 200 Angstroms, then reflectance of incidentelectromagnetic radiation within the first range of NIR radiation may bereduced from about 15% to about 6.2%. In such embodiments, thereflectance of incident electromagnetic radiation within the first rangeof NIR radiation being about 15% corresponds to an embodiment in whichthe second dielectric layer 122 is omitted (e.g., the second thicknesst2 is 0 Angstroms) (not shown). Thus, the second dielectric layer 122increases an overall performance of the pixel sensor 200 a.

In addition, a grid structure 126 overlies the ARC structure 118. Thegrid structure 126 may, for example, comprise a metal grid structureand/or a dielectric grid structure. Further, an upper dielectric layer128 is disposed over the grid structure 126 and the ARC structure 118.In some embodiments, the upper dielectric layer 128 may, for example, beor comprise an oxide such as silicon dioxide, another suitabledielectric material, or any combination of the foregoing. In yet furtherembodiments, the upper dielectric layer 128 comprises a same material asthe third dielectric layer 124.

FIG. 2B illustrates a cross-sectional view of some embodiments of apixel sensor 200 b according to some alternative embodiments of thepixel sensor 200 a of FIG. 2A.

In some embodiments, the second isolation structure 202 comprises apassivation layer 204 and a conductive trench layer 206. The passivationlayer 204 may, for example, be or comprise a dielectric material, suchas silicon dioxide, silicon oxynitride, silicon oxycarbide, anothersuitable dielectric material, or any combination of the foregoing.Further, the passivation layer may continuously extend along theback-side 110 b of the substrate 110. The passivation layer 204 isdisposed between the conductive trench layer 206 and the substrate 110,thereby electrically isolating the conductive trench layer 206 from thesubstrate 110. In further embodiments, the conductive trench layer 206may, for example, be or comprise aluminum, tungsten, copper, anothersuitable conductive material, or any combination of the foregoing.Further, the conductive trench layer 206 may be configured to decreasecross-talk between adjacent image sensor elements disposed within thesubstrate 110. This, in part, further increases a performance of thepixel sensor 200 b. In further embodiments, a thickness of thepassivation layer 204 may be less than the first, second, and/or thirdthicknesses t1, t2, t3. In yet further embodiments, an index ofrefraction of the passivation layer 204 may be equal to the third indexof refraction of third dielectric layer 124. In various embodiments, theindex of refraction of the passivation layer 204 may be less than thefirst index of refraction of the first dielectric layer 120 and/or maybe less than the second index of refraction of the second dielectriclayer 122.

FIG. 2C illustrates a cross-sectional view of some embodiments of apixel sensor 200 c according to some alternative embodiments of thepixel sensor 200 a of FIG. 2A.

A reflector 208 underlies the image sensor element 112 and is separatedfrom the front-side 110 f of the substrate by the interconnectdielectric structure 104. In some embodiments, the reflector 208comprises of a metal material (e.g., aluminum, tungsten, copper, anothermetal material, or any combination of the foregoing). The reflector 208is configured to reflect incident electromagnetic radiation disposed onthe back-side 110 b of the substrate 110 that passes through thefront-side 102 f of the substrate 110 back to the image sensor element112. This, in part, further increases the QE of the image sensor element112, thereby increasing the performance of the pixel sensor 200 c. Inyet further embodiments, the reflector 208 may be comprised of a firstmetal material that is different than a second metal material theconductive wires 106 are comprised of.

FIG. 2D illustrates a cross-sectional view of some embodiments of apixel sensor 200 d according to some alternative embodiments of thepixel sensor 200 a of FIG. 2A.

The substrate 110 may be, for example, a bulk substrate (e.g., a bulksilicon substrate), a silicon-on-insulator (SOI) substrate, P dopedsilicon, N doped silicon, or another suitable material. In someembodiments, the substrate 110 is lightly doped with dopants of a firstconductivity type (e.g., P-type). In various embodiments, the imagesensor element 112 is configured as a single photon avalanche diode(SPAD) which can detect incident radiation with very low intensities(e.g., a single photon). In further embodiments, the image sensorelement 112 may, for example, be used in a near IR (NIR) direct-time offlight (D-TOF) application. In some embodiments, the active layer 114comprises the second material (e.g., germanium) that is lightly dopedwith dopants of the first conductivity type. The active layer 114 maycomprise a first deep well 214 of the first conductivity type and afirst heavily doped region 212 of a second conductivity type (e.g.,N-type) opposite to the first conductivity type. The first deep well 214is disposed above the first heavily doped region 212. A multiplicationjunction region is formed at an interface between the first heavilydoped region 212 and the first deep well 214. In some embodiments, thefirst deep well 214 is vertically spaced from the first heavily dopedregion 212 (not shown) such that the multiplication junction region isformed at an interface between the first heavily doped region 212 andthe active layer 114.

In some embodiments, the image sensor element 112 further comprises asecond deep well 210 heavily doped with dopants of the firstconductivity type. The second deep well 210 extends from the front-side110 f of the substrate to a point above the first heavily doped region212. In some embodiments, the second deep well 210 is configured as aguard ring to prevent premature edge break down of the image sensorelement 112 in the SPAD configuration. Further, a buffer layer 209 isdisposed between the active layer 114 and the substrate 110. The bufferlayer 209 may comprise the same dopants and doping concentration as theactive layer 114. In some embodiments, the buffer layer 209 is omittedsuch that the active layer 114 directly contacts the substrate 110 (notshown). In some embodiments, dopants of the first conductivity type areP-type (e.g., boron, some other suitable P-type dopants, or anycombination of the foregoing) and dopants of the second conductivitytype are N-type (e.g., arsenic, phosphorus, some other suitable N-typedopants, or any combination of the foregoing), or vice versa.

In some embodiments, during operation in the SPAD configuration, theimage sensor element 112 is reverse biased above its breakdown voltage,and incident photons (e.g., wavelengths within the range of nearinfrared (NIR) radiation) strike the image sensor element 112 togenerate charge carriers. The photon-generated charge carries move tothe multiplication junction region and trigger an avalanche current thatamplifies the signals generated by the photons so that they are easierto detect. In some embodiments, a doping type and/or concentration ofthe first deep well 214 can be configured to adjust the breakdownvoltage of the image sensor element 112 in the SPAD configuration. Inyet further embodiments, conductive features (e.g., conductive vias 108and/or conductive wires 106) within the interconnect structure 102 areelectrically coupled to doped regions within the active layer 114 tofacilitate readout of signals generated by the photons.

The second material (e.g., germanium) of the active layer 114 isselected such that the image sensor element 112 has high sensitivity toelectromagnetic radiation having wavelengths within the range of NIRradiation. This increases a quantum efficiency (QE) of the image sensorelement 112 when receiving wavelengths within the range of NIRradiation. However, as the wavelength of the incident electromagneticradiation increases, the QE of the image sensor element 112 maydecrease. For example, if the wavelength of incident electromagneticradiation is about 940 nm, 1310 nm, and/or 1550 nm, then the QE of theimage sensor element 112 may be about 86%, 50%, and/or 32%,respectively. Further, in some embodiments, in order to mitigate damageto the human eye, the wavelength of incident electromagnetic radiationtransmitted by an NIR light source for a D-TOF application may belimited to wavelengths within a first range including about 1400 nm toabout 2600 nm. Thus, the ARC structure 118 is configured to decreasereflection away from the image sensor element 112 when receivingwavelengths within the first range of wavelengths, thereby mitigating aneffect of the lower QE of the image sensor element 112 and increasing anoverall performance of the pixel sensor 200 d in D-TOF applications.

FIG. 2E illustrates a cross-sectional view of some embodiments of apixel sensor 200 e according to some alternative embodiments of thepixel sensor 200 a of FIG. 2A.

In some embodiments, the image sensor element 112 comprises a dopedregion 216 of the substrate 110. In various embodiments, regions of thesubstrate 110 adjacent to and/or contacting the doped region 216comprise a first doping type (e.g., p-type dopants) and the doped region216 comprises a second doping type (e.g., n-type) different from thefirst doping type. In some embodiments, the first doping type is p-typeand the second doping type is n-type, or vice versa. The image sensorelement 112 is configured to generate electrical signals fromelectromagnetic radiation within a range of frequencies. In someembodiments, the range of frequencies may, for example, include visiblelight (e.g., electromagnetic radiation with wavelengths in a range ofabout 400 nm to about 700 nm). It will be appreciated that the imagesensor element 112 being configured to generate electrical signals fromother frequency wavelength values is also within the scope of thedisclosure.

FIG. 2F illustrates a cross-sectional view of some embodiments of apixel sensor 200 f according to some alternative embodiments of thepixel sensor 200 a of FIG. 2A.

A light filter 218 (e.g., a color filter, an infrared (IR) filter, etc.)overlies the ARC structure 118 and is disposed laterally betweensidewalls of the grid structure 126. The light filter 218 is configuredto transmit specific wavelengths of incident radiation. For example, thelight filter 218 may transmit radiation having wavelengths within afirst range while blocking radiation having wavelengths within a secondrange that is different from the first range. Further, a plurality ofmicro-lenses 220 is disposed over the light filter 218 and the gridstructure 126. The micro-lenses 220 are configured to focus incidentelectromagnetic radiation towards the substrate 110, thereby increasingthe QE of the image sensor element 112. In yet further embodiments, theimage sensor element 112 may be configured as the image sensor element112 of the pixel sensor 200 e of FIG. 2E.

FIG. 3 illustrates a cross-sectional view of some embodiments of anintegrated circuit (IC) 300 comprising a first IC die 301 underlying asecond IC die 303, where the second IC die 303 includes an ARC structure118 overlying a plurality of image sensor elements 112 a-b.

As illustrated in FIG. 3 , the first IC die 301 includes a lowerinterconnect structure 307 overlying a lower substrate 302. The lowersubstrate 302 may, for example, be a bulk substrate (e.g., a bulksilicon substrate), a silicon-on-insulator (SOI) substrate, P dopesilicon, N doped silicon, another suitable material, or any combinationof the foregoing. The lower interconnect structure 307 includes theinterconnect dielectric structure 104, the plurality of conductive wires106, and the plurality of conductive vias 108. The lower interconnectstructure 307 is configured to electrically couple semiconductor devices305 disposed on and/or within the lower substrate 302 to each other,other devices (e.g., the image sensor elements 112 a-b), and/or dopedregions disposed within the lower substrate 302. In some embodiments,the semiconductor devices 305 may be configured as transistors, pixeldevices (e.g., source-follower transistors, row-select transistors,reset transistors, etc.), capacitors, other semiconductor devices, orany combination of the foregoing. In yet further embodiments, the firstIC die 301 is configured as an application-specific integrated circuit(ASIC), where the semiconductor devices 305 are configured as ASICdevices

The first and second IC dies 301, 303 meet at a bond interface between afirst bond structure 308 and a second bond structure 316. The first andsecond bond structures 308, 316 each include a bond dielectric structure312, bond etch stop layer 310, redistribution vias 313, andredistribution wires 314. The first and second bond structures 308, 316are configured to facilitate bonding the second IC die 303 to the firstIC die 301 and electrically coupling the interconnect structure 102 tothe lower interconnect structure 307. This, in part, facilitates theimage sensor elements 112 a-b being electrically coupled to thesemiconductor devices 305 by way of the interconnect structure 102 andthe lower interconnect structure 307.

Further, the second IC die 303 includes a plurality of image sensorelements 112 a-b disposed within the substrate 110 and laterally offsetfrom an upper bond pad structure 328. In some embodiments, each of theimage sensor elements 112 a-b comprise the active layer 114 and may beconfigured as the image sensor element 112 of FIGS. 1, 2A-2D, or 2F. Inyet further embodiments, each of the image sensor elements 112 a-b maybe configured as the image sensor element 112 of FIG. 2E, such that eachimage sensor element 112 a-b comprises the doped region (216 of FIG.2E). The ARC structure 118 overlies the back-side 110 b of the substrate110 and is configured to increase a QE of the image sensor elements 112a-b. In some embodiments, the ARC structure 118 includes the passivationlayer 204 disposed between the back-side 110 b of the substrate 110 andthe first dielectric layer 120. In yet further embodiments, thepassivation layer 204 is disposed between the substrate 110 and thesecond isolation structure 202 (not shown) (e.g., see FIG. 2B).

The grid structure 126 overlies the ARC structure 118. In someembodiments, the grid structure 126 may include a first grid layer 321and a second grid layer 322 overlying the first grid layer 321. Thefirst and second grid layers 321, 322 may, for example, each be orcomprise a conductive material, such as tungsten, aluminum, copper, acombination of the foregoing, or the like. In further embodiments, thefirst and second grid layers 321, 322 may be or comprise a conductivematerial or a dielectric material. For example, the first grid layer 321may be or comprise a conductive grid structure (e.g., comprisingtungsten, aluminum, copper, another conductive material, etc.)configured to direct incident radiation towards the image sensorelements 112 a-b and the second grid layer 322 may be or comprise adielectric grid structure configured to achieve total internalreflection (TIR) with the upper dielectric layer 128, or vice versa.This may increase a QE of the image sensor elements 112 a-b. A pluralityof light filters 218 overlies the grid structure 126 and a plurality ofmicro-lenses overlies the plurality of light filters 218. An upper etchstop layer 330 lines an opening 336 that exposes an upper surface of theupper bond pad structure 328. In some embodiments, the upper etch stoplayer 330 may, for example, be or comprise silicon nitride, siliconcarbide, silicon oxynitride, silicon oxycarbide, another suitabledielectric material, or any combination of the foregoing. In someembodiments, a bond pad (not shown) is disposed within the opening 336and overlies the upper bond pad structure 328. The bond pad isconfigured to electrically couple the first and/or second IC dies 301,303 to another integrated circuit (not shown).

FIGS. 4-11 illustrate cross-sectional views 400-1100 of some embodimentsof a method of forming a pixel sensor having an anti-reflective coating(ARC) structure overlying an image sensor element according to thepresent disclosure. Although the cross-sectional views 400-1100 shown inFIGS. 4-11 are described with reference to a method, it will beappreciated that the structures shown in FIGS. 4-11 are not limited tothe method but rather may stand alone separate of the method.Furthermore, although FIGS. 4-11 are described as a series of acts, itwill be appreciated that these acts are not limiting in that the orderof the acts can be altered in other embodiments, and the methodsdisclosed are also applicable to other structures. In other embodiments,some acts that are illustrated and/or described may be omitted in wholeor in part.

As illustrated in the cross-sectional view 400 of FIG. 4 , a substrate110 is provided and a first isolation structure 116 is formed within thesubstrate 110. In some embodiments, the substrate 110 may, for example,be a bulk substrate (e.g., a bulk silicon substrate), asilicon-on-insulator (SOI) substrate, or some other suitable substrate.In further embodiments, the first isolation structure 116 may be formedby selectively etching the substrate 110 to form a trench in thesubstrate 110, and subsequently filing (e.g., by chemical vapordeposition (CVD), physical vapor deposition (PVD), atomic layerdeposition (ALD), or another suitable deposition or growth process) thetrench with a dielectric material. In yet further embodiments, thesubstrate 110 is selectively etched by forming a masking layer (notshown) over a front-side 110 f of the substrate 110, and subsequentlyexposing the substrate 110 to one or more etchants configured toselectively remove unmasked portions of the substrate 110. In variousembodiments, the dielectric material may, for example, be or comprise anoxide (e.g., silicon dioxide), a nitride (e.g., silicon nitride), acarbide (e.g., silicon carbide), another suitable dielectric material,or any combination of the foregoing.

As illustrated in the cross-sectional view 500 of FIG. 5 , an imagesensor element 112 is formed within the substrate 110. In someembodiments, a process for forming the image sensor element 112includes: selectively etching the substrate 110 to define an openingthat extends into the front-side 110 f of the substrate 110; depositingan active material (e.g., germanium) within the opening; and performinga planarization process (e.g., a chemical mechanical planarization (CMP)process) into the active material, thereby forming an active layer 114within the substrate 110. In further embodiments, a buffer layer (notshown) is selectively grown within the opening before forming the activelayer 114, such that the buffer layer is disposed between the activelayer 114 and the substrate 110 (e.g., see the buffer layer 209 of FIG.2D). In yet further embodiments, the buffer layer and/or the activematerial may, for example, respectively be formed by molecular-beamepitaxy (MBE), vapor phase epitaxy (VPE), liquid-phase epitaxy (LPE),some other suitable epitaxial process, some other suitable deposition orgrowth process, or any combination of the foregoing. In addition, one ormore forming processes (e.g., including selective ion implantationprocesses, or other suitable processing steps) may be performed todefine well region(s), doped region(s), or other suitable regions and/orstructures within the active layer 114. For example, the one or moreforming processes may be performed to form the second deep well 210, thefirst heavily doped region 212, and/or the first deep well 214 of FIG.2D in the active layer 114.

In yet alternative embodiments, a process for forming the image sensorelement 112 may include performing a selective ion implantation processinto the substrate 110 to form a doped region (not shown) (e.g., see thedoped region 216 of FIG. 2E) within the substrate 110. The doped regionmay be disposed laterally between sidewalls of the first isolationstructure 116. In such embodiments, the substrate 110 comprises a firstdoping type (e.g., p-type) laterally adjacent to the doped region, wherethe doped region comprises a second doping type (e.g., n-type) oppositeto the first doping type. In such embodiments, the active layer 114 isomitted and the image sensor element 112 is configured as the imagesensor element 112 of FIG. 2E.

In addition, as illustrated in the cross-sectional view 500 of FIG. 5 ,after forming the image sensor element 112, a thinning process isperformed on the back-side 110 b of the substrate 110 to reduce aninitial thickness Ti of the substrate 110 to a thickness Ts. Thethickness Ts is defined between the front-side 110 f of the substrate110 and the back-side 110 b of the substrate 110. In some embodiments,the thinning process may include performing a mechanical grindingprocess, a CMP process, another suitable thinning process, or anycombination of the foregoing.

As illustrated in the cross-sectional view 600 of FIG. 6 , aninterconnect structure 102 is formed over the front-side 110 f of thesubstrate 110. The interconnect structure 102 includes an interconnectdielectric structure 104, a plurality of conductive wires 106, aplurality of conductive vias 108, and a reflector 208. In someembodiments, the interconnect dielectric structure 104 may, for example,be or comprise an oxide (e.g., silicon dioxide), a nitride (e.g.,silicon nitride), a low-k dielectric material, another suitabledielectric material, or any combination of the foregoing. Theinterconnect dielectric structure 104 may be formed by one or moredeposition processes (e.g., CVD, PVD, ALD, or another suitabledeposition or growth process). The plurality of conductive wires 106and/or the plurality of conductive vias 108 may, for example, be formedby a single damascene process, a dual damascene process, or anothersuitable formation process. Further, the reflector 208 may be formedconcurrently with at least one layer of the conductive wires 106 and/orthe conductive vias 108. In some embodiments, the conductive wires 106and/or the conductive vias 108 may, for example, respectively be orcomprise aluminum, copper, titanium nitride, tantalum nitride,ruthenium, another suitable conductive material, or any combination ofthe foregoing. In yet further embodiments, the reflector 208 may, forexample, be or comprise aluminum, tungsten, copper, another metalmaterial, or any combination of the foregoing.

As illustrated by the cross-sectional view 700 of FIG. 7 , the structureof FIG. 6 is flipped and a patterning process is performed into theback-side 110 b of the substrate 110, thereby forming a deep trenchisolation (DTI) opening 702. In some embodiments, the patterning processincludes: forming a masking layer (not shown) over the back-side 110 bof the substrate 110; exposing unmasked regions of the substrate 110 toone or more etchants, thereby forming the DTI opening 702; andperforming a removal process to remove the masking layer.

As illustrated by the cross-sectional view 800 of FIG. 8 , a secondisolation structure 202 is formed over the back-side 110 b of thesubstrate 110, thereby filling the DTI opening (702 of FIG. 7 ). Invarious embodiments, the second isolation structure 202 may beconfigured as a DTI structure and/or may include a passivation layer 204and a conductive trench layer 206. In some embodiments, a process forforming the second isolation structure 202 includes: depositing (e.g.,by CVD, PVD, ALD, or another suitable deposition or growth process) apassivation layer 204 over the substrate 110, where the passivationlayer 204 overlies the back-side 110 b of the substrate 110 and linesthe DTI opening (702 of FIG. 7 ); depositing (e.g., by CVD, PVD,electroless plating, sputtering, electro plating, or another suitabledeposition or growth process) a conductive material over the back-side110 b of the substrate 110, where the conductive material overlies thesubstrate 110 and fills the DTI opening (702 of FIG. 7 ); and performinga planarization process (e.g., a CMP process) into the conductivematerial and/or the passivation layer 204, thereby forming theconductive trench layer 206 and the second isolation structure 202. Insome embodiments, the passivation layer 204 may, for example, be orcomprise a dielectric material, such as silicon dioxide, siliconoxynitride, silicon oxycarbide, another suitable dielectric material, orany combination of the foregoing. In further embodiments, the conductivetrench layer 206 may, for example, be or comprise aluminum, tungsten,copper, another suitable conductive material, or any combination of theforegoing.

As illustrated by the cross-sectional view 900 of FIG. 9 , ananti-reflective coating (ARC) structure 118 is formed over the back-side110 b of the substrate 110. In some embodiments, the ARC structure 118includes a first dielectric layer 120, a second dielectric layer 122,and a third dielectric layer 124. In further embodiments, a process forforming the ARC structure 118 includes: depositing (e.g., by CVD, PVD,ALD, or another suitable deposition or growth process) the firstdielectric layer 120 over the substrate 110; depositing (e.g., by CVD,PVD, ALD, or another suitable deposition or growth process) the seconddielectric layer 122 over the first dielectric layer 120; and depositing(e.g., by CVD, PVD, ALD, or another suitable deposition or growthprocess) the third dielectric layer 124 over the second dielectric layer122, thereby forming the ARC structure 118. In some embodiments, thethird dielectric layer 124 is formed by a plasma-enhanced CVD process,an ALD process, high-density plasma CVD, or another suitable growth ordeposition process.

The first dielectric layer 120 has a first index of refraction, thesecond dielectric layer 122 has a second index of refraction, and thethird dielectric layer 124 has a third index of refraction. In someembodiments, the first dielectric layer 120 may, for example, be orcomprise tantalum oxide (e.g., Ta₂O₅), another dielectric material, orany combination of the foregoing and/or the first index of refractionmay be about 2, about 2.06, within a range of about 2 to 2.16, oranother suitable value. In further embodiments, the second dielectriclayer 122 may, for example, be or comprise titanium oxide (e.g., TiO₂),silicon carbide, another suitable dielectric material, or anycombination of the foregoing and/or the second index of refraction maybe about 2.43, within a range of about 2.4 to 2.6, or another suitablevalue. In yet further embodiments, the third dielectric layer 124 may,for example, be or comprise silicon dioxide (e.g., SiO₂), another oxide,high density plasma oxide, another suitable dielectric material, or anycombination of the foregoing and/or the third index of refraction may beabout 1.44, about 1.438, about 1.53, within a range of about 1.438 to1.53, or another suitable value. In various embodiments, the secondindex of refraction is greater than the first index of refraction and/orthe third index of refraction is less than the first index ofrefraction. This, in part, decreases reflection of electromagneticradiation away from the image sensor element 112, thereby increasing aQE of the image sensor element 112.

In some embodiments, the first dielectric layer 120 is formed to a firstthickness t1 that is, for example, about 1,100 Angstroms, within a rangeof about 900 to 1,300 Angstroms, or another suitable thickness value. Infurther embodiments, the second dielectric layer 122 is formed to asecond thickness t2 that is, for example, about 200 Angstroms, within arange of about 10 to 1,000 Angstroms, or another suitable thicknessvalue. In yet further embodiments, the third dielectric layer 124 isformed to a third thickness t3 that is, for example, about 1,300Angstroms, within a range of about 1,100 to 1,500 Angstroms, or anothersuitable thickness value. In various embodiments, the first thickness t1is greater than the second thickness t2, and the first thickness t1 isless than the third thickness t3.

As illustrated by the cross-sectional view 1000 of FIG. 10 , a gridstructure 126 and an upper dielectric layer 128 are formed over the ARCstructure 118. In some embodiments, the grid structure 126 may comprisea metal grid structure and/or a dielectric grid structure. In furtherembodiments, the metal grid structure and/or the dielectric gridstructure may be formed by, for example, CVD, PVD, ALD, sputtering,electroless plating, electro plating, or another suitable growth ordeposition process. Further, after depositing the metal grid structureand/or the dielectric grid structure, a patterning process may beperformed on the metal grid structure and/or the dielectric gridstructure to define an opening. Subsequently, the upper dielectric layer128 may be formed within the opening and over the ARC structure 118.

As illustrated by the cross-sectional view 1100 of FIG. 11 , a lightfilter 218 is formed over the upper dielectric layer 128 and amicro-lens 220 is formed over the light filter 218. The light filter 218is formed of material that allows for the transmission of incidentelectromagnetic radiation (e.g., light) having a specific wavelengthrange, while blocking incident wavelength with another wavelengthoutside of the specified range. In further embodiments, the light filter218 may be formed by CVD, PVD, ALD, sputtering, or the like and/or maybe planarized (e.g., via a chemical mechanical planarization (CMP)process) subsequent to formation. Further, in some embodiments, themicro-lens 220 may be formed by depositing (e.g., by CVD, PVD, etc.) alens material on the light filter 218. A lens template (not shown)having a curved upper surface is patterned above the lens material. Themicro-lens 220 is then formed by selectively etching the lens materialaccording to the lens template.

FIG. 12 illustrates a method 1200 of forming a pixel sensor having ananti-reflective coating (ARC) structure overlying an image sensorelement according to the present disclosure. Although the method 1200 isillustrated and/or described as a series of acts or events, it will beappreciated that the method is not limited to the illustrated orderingor acts. Thus, in some embodiments, the acts may be carried out indifferent orders than illustrated, and/or may be carried outconcurrently. Further, in some embodiments, the illustrated acts orevents may be subdivided into multiple acts or events, which may becarried out at separate times or concurrently with other acts orsub-acts. In some embodiments, some illustrated acts or events may beomitted, and other un-illustrated acts or events may be included.

At act 1202, a first isolation structure is formed into a front-side ofa substrate, where the substrate comprises a first material. FIG. 4illustrates a cross-sectional view 400 corresponding to some embodimentsof act 1202.

At act 1204, an image sensor element is formed in the substrate, wherethe image sensor element has an active layer comprising a secondmaterial different from the first material. FIG. 5 illustrates across-sectional view 500 corresponding to some embodiments of act 1204.

At act 1206, an interconnect structure is formed along a front-side ofthe substrate. FIG. 6 illustrates a cross-sectional view 600corresponding to some embodiments of act 1206.

At act 1208, a second isolation structure is formed into a back-side ofthe substrate, where the second isolation structure extends from theback-side of the substrate to the first isolation structure. FIGS. 7 and8 illustrate cross-sectional views 700 and 800 corresponding to someembodiments of act 1208.

At act 1210, an anti-reflective coating (ARC) structure is formed overthe back-side of the substrate. The ARC structure includes a firstdielectric layer with a first index of refraction overlying thesubstrate, a second dielectric layer with a second index of refractionoverlying the first dielectric layer, and a third dielectric layer witha third index of refraction overlying the second dielectric layer.Further, the second index of refraction is greater than the third indexof refraction. FIG. 9 illustrates a cross-sectional view 900corresponding to some embodiments of act 1210.

At act 1212, a grid structure and an upper dielectric layer are formedover the ARC structure. FIG. 10 illustrates a cross-sectional view 1000corresponding to some embodiments of act 1212.

At act 1214, a light filter is formed over the upper dielectric layerand a micro-lens is formed over the light filter. FIG. 11 illustrates across-sectional view 1100 corresponding to some embodiments of act 1214.

Accordingly, in some embodiments, the present disclosure relates to ananti-reflective coating (ARC) structure overlying an image sensorelement, where the ARC structure includes a first dielectric layer witha first index of refraction overlying the image sensor element, a seconddielectric layer with a second index of refraction overlying the firstdielectric layer, and a third dielectric layer with a third index ofrefraction overlying the second dielectric layer. The second index ofrefraction is greater than the first index of refraction and the firstindex of refraction is greater than the third index of refraction.

In some embodiments, the present application provides a pixel sensorincluding: a substrate having a front-side opposite a back-side; animage sensor element comprising an active layer disposed within thesubstrate, wherein the active layer comprises germanium; and ananti-reflective coating (ARC) structure overlying the back-side of thesubstrate, wherein the ARC structure includes a first dielectric layeroverlying the back-side of the substrate, a second dielectric layeroverlying the first dielectric layer, and a third dielectric layeroverlying the second dielectric layer, wherein a first index ofrefraction of the first dielectric layer is less than a second index ofrefraction of the second dielectric layer, and wherein a third index ofrefraction of the third dielectric layer is less than the first index ofrefraction.

In some embodiments, the present application provides an integratedcircuit (IC) including: a first IC die including a first substrate and afirst interconnect structure overlying the first substrate; a second ICdie overlying the first IC die, wherein the second IC die includes asecond substrate and a second interconnect structure underlying thesecond substrate, wherein the first and second IC dies contact at a bondinterface between the first and second interconnect structures; aplurality of image sensor elements disposed within the second substrate;a grid structure overlying the plurality of image sensor elements,wherein each image sensor element is spaced laterally between sidewallsof the grid structure; and an anti-reflective coating (ARC) structuredisposed between the second substrate and the grid structure, whereinthe ARC structure includes a first dielectric layer, a second dielectriclayer, and a third dielectric layer each with an index of refractionthat is different from one another, wherein the second dielectric layeroverlies the first dielectric layer and the third dielectric layeroverlies the second dielectric layer, and wherein the first dielectriclayer comprises a first metal oxide and the second dielectric layercomprises a second metal oxide different from the first metal oxide.

In some embodiments, the present application provides a method forforming a pixel sensor, the method includes: forming a first isolationstructure into a front-side of a substrate; forming an image sensorelement within the substrate such that the image sensor element includesan active layer spaced laterally between sidewalls of the firstisolation structure, wherein the substrate comprises a first materialand the active layer comprises a second material different from thefirst material; forming an interconnect structure along the front-sideof the substrate; and forming an anti-reflecting coating (ARC) structureover a back-side of the substrate such that the ARC structure includes afirst dielectric layer overlying the back-side of the substrate, asecond dielectric layer overlying the first dielectric layer, and athird dielectric layer overlying the second dielectric layer, wherein afirst index of refraction of the first dielectric layer is less than asecond index of refraction of the second dielectric layer, and wherein athird index of refraction of the third dielectric layer is less than thefirst index of refraction.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A pixel sensor, comprising: a substrate having afront-side opposite a back-side; an image sensor element comprising anactive layer disposed within the substrate, wherein the active layercomprises germanium; and an anti-reflective coating (ARC) structureoverlying the back-side of the substrate, wherein the ARC structurecomprises a first dielectric layer overlying the back-side of thesubstrate, a second dielectric layer overlying the first dielectriclayer, and a third dielectric layer overlying the second dielectriclayer, wherein a first index of refraction of the first dielectric layeris less than a second index of refraction of the second dielectriclayer, and wherein a third index of refraction of the third dielectriclayer is less than the first index of refraction.
 2. The pixel sensor ofclaim 1, wherein the first dielectric layer, the second dielectriclayer, and the third dielectric layer each comprise a dielectricmaterial different from one another.
 3. The pixel sensor of claim 1,wherein a thickness of the first dielectric layer is greater than athickness of the second dielectric layer, and wherein a thickness of thethird dielectric layer is greater than the thickness of the firstdielectric layer.
 4. The pixel sensor of claim 1, wherein the imagesensor element is configured to generate electrical signals from nearinfrared (NIR) radiation.
 5. The pixel sensor of claim 1, wherein thefirst dielectric layer comprises tantalum oxide, the second dielectriclayer comprises titanium oxide or silicon carbide, and the thirddielectric layer comprises silicon dioxide.
 6. The pixel sensor of claim1, further comprising: an isolation structure disposed within thesubstrate, wherein the isolation structure extends from the back-side ofthe substrate to the front-side of the substrate, and wherein the imagesensor element is spaced laterally between sidewalls of the isolationstructure.
 7. The pixel sensor of claim 1, further comprising: aninterconnect structure disposed along the front-side of the substrate,wherein the interconnect structure includes an interconnect dielectricstructure, a plurality of conductive vias, and a plurality of conductivewires; and a reflector disposed within the interconnect dielectricstructure and directly underlying the image sensor element.
 8. The pixelsensor of claim 1, further comprising: a grid structure overlying theARC structure; a light filter overlying the grid structure; and amicro-lens overlying the light filter.
 9. The pixel sensor of claim 1,wherein the substrate comprises a first material different fromgermanium.
 10. An integrated circuit (IC), comprising: a first IC diecomprising a first substrate and a first interconnect structureoverlying the first substrate; a second IC die overlying the first ICdie, wherein the second IC die comprises a second substrate and a secondinterconnect structure underlying the second substrate, wherein thefirst and second IC dies contact at a bond interface between the firstand second interconnect structures; a plurality of image sensor elementsdisposed within the second substrate; a grid structure overlying theplurality of image sensor elements, wherein each image sensor element isspaced laterally between sidewalls of the grid structure; and ananti-reflective coating (ARC) structure disposed between the secondsubstrate and the grid structure, wherein the ARC structure comprises afirst dielectric layer, a second dielectric layer, and a thirddielectric layer each with an index of refraction that is different fromone another, wherein the second dielectric layer overlies the firstdielectric layer and the third dielectric layer overlies the seconddielectric layer, and wherein the first dielectric layer comprises afirst metal oxide and the second dielectric layer comprises a secondmetal oxide different from the first metal oxide.
 11. The IC of claim10, wherein the first metal oxide is tantalum oxide and the second metaloxide is titanium oxide.
 12. The IC of claim 10, wherein a first indexof refraction of the first dielectric layer is less than a second indexof refraction of the second dielectric layer, wherein a third index ofrefraction of the third dielectric layer is less than the second indexof refraction.
 13. The IC of claim 12, wherein the ARC structure furthercomprises a passivation layer disposed between the first dielectriclayer and the second substrate, wherein an index of refraction of thepassivation layer is equal to the third index of refraction.
 14. The ICof claim 13, wherein a thickness of the third dielectric layer isgreater than a thickness of second dielectric layer, wherein thethickness of the second dielectric layer is greater than a thickness ofthe passivation layer.
 15. The IC of claim 12, wherein the second indexof refraction is within a range of about 2.4 to 2.6 and the third indexof refraction is less than about 1.55.
 16. The IC of claim 10, furthercomprising: a first isolation structure extending from a front-sidesurface of the second substrate to a first point above the front-sidesurface, wherein each image sensor element is spaced laterally betweensidewalls of the first isolation structure; and a second isolationstructure extending from a back-side surface of the second substrate toa second point below the back-side surface, wherein the second point isbelow the first point such that the second isolation structure contactsthe first isolation structure.
 17. An integrated circuit (IC),comprising: a first IC die comprising a first substrate and a firstinterconnect structure overlying the first substrate; a second IC dieoverlying the first IC die, wherein the second IC die comprises a secondsubstrate, a second interconnect structure underlying the secondsubstrate, and a plurality of image sensor elements disposed in thesecond substrate, wherein the first and second interconnect structuresare disposed between the first and second substrates, wherein the imagesensor elements respectively comprise a germanium layer disposed in thesecond substrate; and an anti-reflective coating (ARC) structuredisposed on a back-side surface of the second substrate, wherein the ARCstructure comprises a first dielectric layer, a second dielectric layer,and a third dielectric layer, wherein the second dielectric layer isdisposed between the first and third dielectric layers, wherein athickness of the first dielectric layer is greater than a thickness ofthe second dielectric layer and is less than a thickness of the thirddielectric layer.
 18. The IC of claim 17, wherein materials of thefirst, second, and third dielectric layers are different from oneanother.
 19. The IC of claim 17, wherein the ARC structure furthercomprises a passivation layer disposed between the first dielectriclayer and the second substrate, wherein a thickness of the passivationlayer is less than the thickness of the second dielectric layer.
 20. TheIC of claim 17, further comprising: an upper bond pad structure disposedon the second interconnect structure and laterally offset from theplurality of image sensor elements; and a liner layer extending alongopposing sidewalls of the ARC structure to the upper bond pad structure.